The math of the Graham number is relatively straightforward. Graham believed that the price-to-earnings (P/EPS) ratio should be no more than 15. He also believed that the price-to-book value (P/BVPS) ratio should be no more than 1.5.

From that, Graham proposed that -- as a rule of thumb -- the product of the two should not be more than 22.5. In other words, (P/EPS of 15) x (P/BVPS of 1.5) = 22.5.